This invention relates to scanning display devices, and more particularly to light sources and scanners for scanning display devices.
In a typical scanning display device light is emitted from a light source, collimated through a lens, then passed through a scanner. The scanner deflects the light along a scanning pattern. The scanned light is directed toward a viewing screen, or in a retinal display, toward a viewer's eye. An image is perceived by a viewer's eye, either from viewing the screen or from the direct scanning of light on the eye.
For a retinal display, an eyepiece directs the light toward the viewer's eye. An "exit pupil" occurs shortly beyond the eyepiece in an area where a viewer's eye pupil is to be positioned. A viewer looks into the eyepiece to perceive the image. A small exit pupil allows persons who ordinarily wear corrective lenses to see the scanned image clearly without glasses. But generally, a small exit pupil is disadvantageous because one must carefully align the eye with the exit pupil to see the image. Slight movement of the eye, such as into one's peripheral viewing area, would cause the image to disappear. Accordingly, it is generally desirable to have large exit pupils.
The size of the exit pupil is determined by the aperture stop of the display and the optical magnification of components along the light path between the optical stop and the viewer's eye. Typically, the horizontal scanner of a raster scanning display is the limiting aperture. The exit pupil diameter is equal to the horizontal scanner aperture times the tangent of half the optical scan angle of the horizontal scanner divided by the tangent of half the field of view angle. Scanners that scan at high horizontal scanning speeds have small apertures, small scan angles or both. This results in a small exit pupil. To increase the exit pupil size, it is desirable to have a larger aperture. To achieve a large aperture a larger scanning mirror may be used. The drawback of a larger mirror is the increase in system size and weight, and the additional power required to move the mirror. Accordingly, another solution is desired for increasing exit pupil size while minimizing system size and weight.